Automotive manufacturers and the National Highway Transportation Safety Association are investigating methods to disable vehicle air bags in situations where they may cause more harm than good. Typically, airbags have been developed to deploy with enough force to restrain a 175 lb. adult in a high velocity crash. Deployment of the same air bags when children are seat occupants may cause serious injury due to the force generated upon inflation of the bag.
As a result, seat weight sensors and systems are being developed in an attempt to determine when the passenger seat occupant is a child. Such systems should identify when the occupant is small, or even when a child is in a rear facing infant seat, a forward facing child seat or a booster seat. Occupant weight measurement when a child seat is present is further complicated by the downward force applied to the child seat by the tension of a seat belt. When a child seat is strapped tightly, the seat belt forces the child seat into the vehicle seat and can often artificially increase the measured weight, which may lead to air bag deployment when children or infants are present in the seat.
Therefore, a variety of methods have been used for seat belt tension measurement. Copending U.S. Provisional Application Ser. No. 60/067,071 entitled "Villari Effect Seat Belt Tension Sensor", and copending U.S. Provisional Application Ser. No. 60/070,319 entitled "Compressive Villari Effect Seat belt Tension Sensor", both assigned to the assignee of the instant invention, disclose two seat belt tension measurement systems utilizing sensors that operate on the principle known as the Villari effect. The Villari effect refers to the change of magnetic permeability responsive to compression or tensile stresses of certain materials with magnetostrictive properties. By measuring the magnetic field strength in magnetostrictive material placed in line with a seat belt mechanism, for example in a seat belt latch or a seat belt retractor, the relative tension in the belt may be calculated.
Tension measurement mechanisms have also been incorporated in the buckle of the seat belt. In one embodiment, a sliding buckle is biased with a spring. When the belt is under heavy tension, the buckle pulls forward to control a switch that provides feedback to a vehicle processor.
The aforementioned seat belt tension measurement methods suffer from a number of disadvantages. Initially, a great number of additional parts are required for seat belt retractors or buckle configurations thereby adding complexity and cost to vehicle assembly and providing considerable difficulty in retrofitting existing vehicles. Additionally, many prior art seat belt tension systems provide only for a threshold level of tension detection.
The present invention may be used to detect whether the seat belt is under high tension thereby indicating that an infant seat or another inanimate object is belted into the seat. The instant invention can be used in conjunction with a seat weight sensor to determine whether an airbag should be deployed for a given occupant. Additionally, the instant invention provides a continuous measurement of seat belt tension in contrast to the threshold detection of some prior art belt tension devices.